remember the milk is an absolutely brilliant task management service.you can create a free account, set tasks and reminders. you can set due tasks through Twitter as well. you can recieve reminders via email, SMS and instant messenger (AIM, GAdu-Gadu, Google Talk, ICQ, Jabber, MSN, Skype and Yahoo! are all supported).

Work together to get things done.Share, send and publish tasks and lists with your contacts or the world. You could Group your contacts. 

Groups are simply groupings of your contacts. You can give them names like Friends or Enemies.

Putting your contacts into groups makes it easier when you want to share a task with multiple people. For example, instead of individually select the names of everyone in your office, you could just choose to share with your Co-workers group.

Departments/Groups can set up Groups. Client Management teams can setup tasks, deadlines and reminders and add them to the group. All related people in the group, Creatives,Media, Planning, Servicing, suppliers etc, will be automatically reminded of the tasks, as well as the task location, using the Maps. So Servicing people’s chores of reminding and following up on people is reduced.

Work together to get things done.Share, send and publish tasks and lists with your contacts or the world. You could Group your contacts. 

Groups are simply groupings of your contacts. You can give them names like Friends or Enemies.
Putting your contacts into groups makes it easier when you want to share a task with multiple people. For example, instead of individually select the names of everyone in your office, you could just choose to share with your Co-workers group.